In the manufacture of electronic components such as integrated circuits, corrosive gases are used (pure or mixtures) to carry out various operations, particularly the various etching steps. These gases need to be very pure, i.e., their particulate and gaseous contaminants should be as low as possible. These gases are usually flowing from a cylinder located remotely from the point of use through a piping network installed throughout the plant from said gas cabinet to the point of use. To make this piping system, it is necessary to use several individual pipings and various components such as valves, fittings, filters, pressure regulators or the like which are usually welded together. It is known that 316L stainless steel electropolished tubings are corrosion resistant; however welding of these tubings together or with other various components induce localized areas sensitive to corrosion. These areas are of concern regarding both safety and micro contamination aspects. Particularly, it is believed that those localized areas sensitive to corrosion might be a source of micro contamination of the gases flowing through these tubings. A distribution system of gases for semiconductor manufacture is disclosed in the article entitled "HCl Gas Distribution System: The Effect of Surface Finish and Point of Use Purification", R. Diguid et al., July 1993, Solid State Technology, pp. 79-85. Also, the Orbital Tungsten Inert Gas welding process is disclosed in the article entitled "Orbital TIG Welding of Electropolished High Alloy Steel Tubes", by Herbert Geil; Stainless Steel Europe, May 1992.
Corrosion resistance of a stainless steel surface is known (from H. H. Uhlig & R. W. Review, "Corrosion and Corrosion Control" by John Wiley & sons, 3rd Edition, 1985) to be related to the surface enrichment in chromium oxide while on the contrary the presence of superficial iron oxide would have a detrimental effect on corrosion resistance.
The welding bead area is known to be the corrosion sensitive point of a welded assembly of stainless steel parts. The experiments conducted by the inventors indicate that the corrosion occurs mainly in a zone distance between about 2 mm and 10 mm from the bead in the case of electropolished stainless steel tubing of 1 mm thickness. The length of the above zone depends on the energy input from the welding, and the corrosion sensitization of this zone is attributed to modifications of the surface near the welding bead during the welding.
The oxidation reduction properties are controlled by the thermodynamic equilibrium between the metal surface and the gas atmosphere as explained in the Ellingham diagram disclosed in B. J. Reed, "Free Energy of Formation of Binary Compounds", Eds. (1985).
The inventors thus believe that the modification of the surface is related to the oxygen partial pressure of the back shielding gas both directly by chemical surface reaction and indirectly via elemental diffusion phenomena between the base alloy and the modified surface.
The level of oxidetire impurities in the common industrial grade argon creates in the zone of the device heated by the welding a layer composed of a mixture of iron oxide and chromium oxide near and on the welding bead. It is therefore very important to control the level of oxygen containing chemicals such as moisture, oxygen, or carbon dioxide, in the welding back shield gas.
In the Tungsten Inert Gas welding (TIG) process, the gases used as back shielding gases obtainable from the gas supplier ere usually pure argon having a purity which is at least equal to 99.999% (the impurities content is below 10 ppm).
In addition, in the case of tube welding, the source of impurities arises from the gas itself as well as by desorption from the wall of the tube upstream. These impurities are as defined hereabove, essentially oxygen-containing compounds such as H.sub.2 O, O.sub.2, CO.sub.2 or the like.
The usual technique is to decrease the oxygen partial pressure at the welding bead at the time of the welding. To do so, it is known to add hydrogen, but this has some drawbacks, especially when it comes to the safety point of view. (In the microelectronics industry, hydrogen detectors are present in the clean rooms where the welding work is performed). Furthermore, hydrogen can be dissolved in the alloy, which is detrimental to its mechanical and outgassing properties.
It is also known from M. Morin, S. Miyoshi, K. Kawada, and T. Ohmi, "Ultra Clean Welding for High Grade Gas Handling Technology", in Electro Chemical Society, Meeting, Hi. (1993), that manganese particles formed during the welding and deposited downstream (the maximum deposition occurs at about 5 mm from the welding bead) are detrimental to corrosion resistance.
It is also known that corrosion by gases such as HCl or HBr is accelerated in the presence of impurities, particularly H.sub.2 O. Even with high purity HCl or HBr, the metallic surface state will strongly affect the corrosion, particularly when H.sub.2 O is adsorbed at the metal surface prior to corrosive gas exposure of high or low purity.
When making a piping system, e.g., for a semi-conductor plant, between the gas cabinet and the point of use, tubes are usually welded together, then the entire line of piping is purged in order to clean it essentially by removing moisture before using it. However, there is still a problem regarding the use of corrosive gases with which even such a procedure is not very efficient, and the corresponding piping system has to be changed frequently.